First, a frame structure of a wireless communication system is described with reference to FIG. 1. FIG. 1 shows a frame structure of a Long Term Evolution (LTE) system. As shown, one subframe includes 10 subframes each having two slots. A time required to transmit one subframe is defined as a transmission time interval (TTI). For example, one subframe may have a length of 1 millisecond (ms), and one slot may have a length of 0.5 ms.
One slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols. An OFDM symbol may also be referred to as a Single Carrier-Frequency Division Multiple Access (SC-FDMA) symbol or a symbol interval.
One slot includes 7 or 6 OFDM symbols depending on a length of a Cyclic Prefix (CP). In the LTE system, there are a normal CP and an extended CP. When a normal CP is used, one subframe includes 7 OFDM symbols, and when an extended CP is used, one subframe includes 6 OFDM symbols. The extended CP is used in case of a great delay spread.
FIG. 2 shows a slot structure of the LTE system. As shown in FIG. 2, a signal transmitted in each slot may be described by a resource grid consisting of NDLRBNRBsc subcarriers and NDLsymb OFDM symbols. NDLRB represents the number of Resource Blocks (RBs), NDLsc represents the number of subcarriers in one RB, and NDLsymb represents the number of OFDM symbols in one slot.
Next, a positioning method to determine the position of a User Equipment (UE) according to the prior art will be described.
In recent times, there is a growing need for a positioning method to determine the position of a UE for a variety of applications. A Global Positioning System (GPS) based scheme and a terrestrial positioning based scheme are widely known as positioning methods of the UE.
The GPS based scheme measures the position of the UE using satellites. However, the GPS based scheme needs to receive signals from at least 4 satellites and cannot be used indoors.
Meanwhile, the terrestrial positioning based scheme measures the position of the UE location by using a timing difference between signals received from base stations (BSs) and needs to receive signals from at least 3 BSs. Although the terrestrial positioning based scheme has lower positioning capabilities than the GPS based scheme, it can be used in almost all environments. The terrestrial positioning based scheme estimates the position of the UE mainly using a synchronization signal or a reference signal. The terrestrial positioning based scheme is defined as different terms according to standard.
Namely, the terrestrial positioning based scheme is defined as Observed Time Difference Of Arrival (OTDOA) in a UMTS Terrestrial Radio Access Network (UTRAN), Enhanced Observed Time Difference (E-OTD) in a GSM/EDGE Radio Access Network (GERAN), and Advanced Forward Link Trilateration (AFLT) in CDMA 2000.
FIG. 3 shows an exemplary downlink OTDOA, which is one terrestrial positioning based scheme, used in 3GPP. As shown in FIG. 3, since a UE operates a reference clock based on a subframe transmitted by a current serving cell, signals received from neighbor cells have different Time Differences Of Arrival (TDOAs).
A positioning method of a UE using OTDOA is described by way of example. A reference cell may be a serving cell and, if the UE has performed a handover operation, the reference cell may be a serving cell before the handover operation. Alternatively, the reference cell may not be changed irrespective of the handover operation of the UE. The positioning method of the UE may be typically performed through a Common Reference Signal (CRS) or a Primary Synchronization Signal/Secondary Synchronization Signal (PSS/SSS), it may be performed by defining a dedicated Positioning Reference Signal (PRS) for a LoCation Service (LCS). The UE uses reference signals or synchronization signals received from one reference cell and a plurality of neighbor cells to calculate differences between a time required to receive a signal from the one reference cell and times required for receiving signals from the plurality of neighbor cells. The UE transmits the calculated time differences to an Enhanced-Serving Mobile Location Center (E-SMLC). The E-SMLC can then calculate the position of the UE by solving a linearlized equation using the Taylor series expansion.
However, a plurality of cells can participate in determining the position of a UE. In actual implementation, a plurality of cells may have different PRS transmission periods.
If a PRS period varies according to each cell, since a plurality of measurement intervals for determining the position of the UE are present, the time when the UE should report measurement results is uncertain.
As described above, the prior art is problematic in that the time for a UE to report measurement results is uncertain since multiple measurement intervals for determining the position of the UE are present when PRS transmission periods of cells participating in the position determination of the UE differ.